An alternate method was designed to couple a limited quantity of protein to an affinity support when a conventional technique was unsuccessful. This was achieved through the introduction of a small number of sulfhydryl groups to the ligand by reaction with 2-iminothiolane which resulted in a limited number of reactive sites on the protein. Amino groups on an AH-Sepharose 4B matrix were linked to sulfhydryl groups on the ligand using the heterobifunctional agent m-maleimidobenzoyl sulfosuccinimide ester (sulfo-MBS). This method was employed to prepare an affinity support using a cytosolic protein that activates glyceraldehyde-3-phosphate dehydrogenase as a ligand. Monospecific antibody purified from the affinity column recognized only this protein on a Western blot of a cytosolic extract of kidney epithelial cells.